The present invention relates to a method of forming complexes, and more paricularly relates to a method of forming organic charge transfer complexes.
Research has been carried out into formation of charge transfer complexes of low molecular compounds and at the present time many such complexes have been obtained. In such complexes electric charge is exchanged between constituent donor molecules (represented simply by D) supplying electrons and constituent acceptor molecules (represented simply by A) receiving electrons. Some complexes can be formed with high conductivities depending upon the amount of charge transferred.
Among usuful acceptor molecules are TCNQ (tetracyanoquinodimethane) and derivatives thereof, for example; ##STR1##
These molecules have been known to possess large electron affinities and form charge transfer complexes with many donor molecules. Similar molecules are as follow: ##STR2##
The acceptor molecules of this kind are called generally TCNQ-based molecules.
Among usuful donor molecules in turn are, for example: ##STR3##
The donor molecules of this kind are called simply TTF-based molecules. In the molecular structures of TTF-based molecules and TCNQ-based molecules, central atoms are arranged in a plane constituting the backbone of the molecule (called here the molecular plane). A number of .pi. electrons existing in the molecular plane play determinant functions for the characteristics of the substance.
Electron transfer complexes can be obtained by combining suitable donor and acceptor molecules, for example, by combining the above described TTF-based and TCNQ-based molecules such as (TTF).sub.x (TCNQ).sub.y, (TMTTF).sub.x (TCNQ).sub.y, (TSF).sub.x (TCNQ).sub.y and (TMTSF).sub.x (TCNQ).sub.y. Several tens or thousands S/cm of conductivity may be achieved by selecting suitable preparation methods.
In order to form metallic phases having such high conductivity, there are two requirements as follow. First, the ratio between x and y in D.sub.x A.sub.y has to be adjusted at an optimum value by, e.g. eliminating impurities to avoid disturbance of crystalline structure due to the existence of impurities. In the case of (TTF-based molecule).sub.x (TCNQ-based molecule).sub.y, the ratio has to be adjusted so that x:y=1:1 in order to obtain desired black metallic complexes while complexes becomes a semiconductor at x:y=1.3:2 or 1.66:2. Second, donor molecules and acceptor molecules have to be arranged into a "separated laminate" in which donor and acceptor molecules are laminated in different planes which are not parallel with each other as illustrated in FIG. 1(A). In the figure, solid lines represent the molecular planes of respective molecules. When a complex is prepared in this laminate structure, there are formed separate molecular columns consisting of donor molecules and acceptor molecules respectively. The molecular columns form current paths respectively in the complex so that its conductivity can be increased. For this reason, it is very important point for obtaining complexes having high conductivities to construct crystalline structures selectively into the separated laminate.
Several attempts have been made to form charge transfer complexes. Many of them generally utilize liquid state reactions of solvents such as diffusion methods or electric field methods. The preparation time required to form complexes in such conventional methods, however, is very long, for example, several months or longer in the case of the diffusion method. When prepared in such a long time, donor molecules and acceptor molecules are arranged into an overlapping laminate in which the donor molecules and acceptor molecules are arranged in parallel and partially and alternately overlapped, rather than the separated laminate, as illustrated in FIG. 1(B). This is because the overlapping laminate constitutes the stable state of the complex while the separated laminate constitutes only a metastable state.
For example, it has been reported that, even if (TMTSF).sub.x (TCNQ).sub.y was formed by a liquid phase diffusion at x:y= 1:1, only reddish crystals corresponding to the overlapping laminate were formed when a long formation time was spent while black crystals corresponding to the separated laminate were obtained when the formation time was relatively short. From these considerations, it seems preferred to decrease the time spent for the formation of complexes in order to achieve high conductivities.
There are some exceptions that complexes having relatively high conductivities are formed in fact even by conventional liquid phase methods. For example, (TMTTF).sub.x (TCNQ).sub.y can be formed in a metallic phase with black crystalline appearance at x:y=1:1 by a liquid phase method. This is, however, hardly applicable for commercialization (mass-production) since the preparation time is too long, several months or longer in the case of usual liquid diffusion methods as described above. When prepared within a shorter time, the product of complexes assume only the form of microcrsytals which can be utilized for few applications. In addition, the control of complex crystal formation is too difficult to reproduce appropriate separated laminates required to realize sufficient conductivities for applications.